This is not about McCain or (R)epublicans, it is about self-respect. Why do people not have enough self-respect that they have the dignity to treat others they do not agree with with respect? Yes, it seems to appear frequently when "conservatives" appear in front of "liberals," but the fact that it happens tells you a bit about those who are so rude.
Many years ago, and again, Angela Davis gave a commencement speech at UC Berkeley. There were many who did not agree, but no one turned their backs or made rude comments -- and there was some serious disagreement.
Being rude does not mean you are tough, or righteous. It simply means you are rude and lacking in self-respect.